Transfer process in electrography

ABSTRACT

1. A TRANSFER PROCESS FOR PRODUCING A PLURALITY OF COPIES FROM A SINGLE ELECTROSTATIC LATENT IMAGE FORMED ON THE SURFACE OF A SUPPORTING MEMBER COMPRISING THE STEPS OF DEVELOPING THE LATENT IMAGE WITH TONER HAVING THE SAME POLARITY CHARGE AS THE LATENT IMAGE; TRANSFERRING THE DEVELOPED TONER IMAGE FROM THE SUPPORTING MEMBER TO A TRANSFER SHEET WITHOUT SUBSTANTIALLY DISSIPATING THE ELEC TROSTATIC LATENT IMAGE BY PRESSURE-CONTACTING THE SURFACE OF THE SUPPORTING MEMBER AND THE TRANSFER SHEET WITH A PRESSURE WITHING THE RANGE OF FROM 0.1 TO 0.3 KG./CM.2 UTILIZING AN ELECTRICALLY CONDUCTIVE ROLLER, THE TRANSFER SHEET HAVING AN ELECTROSTATIC CAPACITY NOT GREATER THAN THE ELECTROSTATIC CAPACITY OF THE SUPPORTING MEMBER AND HAVING A VOLUME RESISTIVITY WITHIN THE RANGE OF FROM 10**8 TO 10**14 $.CM., THE ELECTRICALLY CONDUCTIVE ROLLER AND THE REAR SURFACE OF THE SUPPORTING MEMBER HAVING SUBSTANTIALLY EQUAL POTENTIALS, AND REPEATING SAID DEVELOPING AND TRANSFERRING STEPS TO PRODUCE A PLURALITY OF COPIES FROM THE SINGLE ELECTROSTATIC LATENT IMAGE.

Oct. 22, 1974 5H0, MATSUMOTO ETAL 3,843,381

TRANSFER PROCESS IN ELETROGRAPHY Filed Dec. 28, 1971 2 Sheets-Sheet 1 .TTITL 4 Fig. 2 5 6 5 6 Tiff? ??9%?ii iie fi 4 13%W51 @HQM MAT$UMT WM TRMESJE'ER PROCESS mm ELETROGRAPHY Filed Dan. 28, 1971 2 sheets she at 2 United States Patent C1 hoe Patented Oct. 22, 1974 3,843,381 TRANSFER PROCESS IN ELECTROGRAPHY Shoji Matsumoto, Neyagawa, Noboru Yonaha, Hirakata, and Tatsuo Aizawa,0saka, Japan, assignors to Mita Industrial Company, Ltd., Osaka, Japan Filed Dec. 28, 1971, Ser. No. 213,023 Claims priority, application Japan, Dec. 29, 1970, 46/121,545 Int. Cl. G03g 13/08, 13/10 US. Cl. 117-175 9 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a transfer process in electrography. More particularly, the invention relates to a transfer process in electrography whereby it is possible to obtain multi-copies of toner image from a single electrostatic latent image.

Conventionally known methods for obtaining multicopies of toner image from a single electrostatic latent image include that, in xerography, wherein a potential of opposite polarity from that of the toner image is applied from the back of the transfer member to attract at least a part of the toner, then another potential of same polarity with that of the toner is applied to return the previously attracted part of the toner onto the photosensitive plate and obtain the first copy on the transfer member, and further the toner image retained on the photosensitive plate is transferred by a similar procedure to produce multi-copies (US. Pat. No. 2,812,709). In another known method, the developed image and transfer member are intimately contacted by conducting rollers, a monopolar transfer potential is applied between the rollers to attract at least a part of the toner onto the transfer member and obtain the first copy, and then the same latent image is re-developed, followed by the similar procedure with gradually increased transfer potential to transfer toner images of substantially identical contrast (Japanese Official Patent Gazette, Publication No. 30,233/ 69).

Moreover, in the former process the toner image adhered to the electrostatic latent image is transferred onto the transfer member dividedly by the repetition of the procedures, and for this reason the clarity and contrast of the transferred images unavoidably deteriorate in the attempt to produce multi-copies. In the latter process the development and transfer with toner particles are repeated with a single electrostatic latent image, wherein it is necessary to apply carefully controlled transfer potential for transferring the toner particles. Thus not only the transfer operation is complex, but also a disturbance in electrostatic latent image unavoidably takes place during the repetition of development and transfer due to the applying of transfer potential.

As above-described, in all of the presently practiced repetitive transfer processes from one electrostatic latent image, applying of transfer potential and control thereof are necessary in that it was believed impossible to effect repetitive transfer of toner images from a single electrostatic latent image without the application of such transfer potential.

Whereas, we discovered quite unexpectedly that multicopies of toner image can be formed from a single electrostatic latent image without the application of transfer potential when the surface carrying the electrostatic latent image is repulsion-developed with specific toner and a specific transfer sheet, hereinafter described in detail is pressure-contacted with the surface by a conducting roller, and the foregoing procedures are repeated.

The present invention is generally characterized in a transfer process for producing a plurality of copies from a single electrostatic latent image formed on the surface of a supporting member comprising the steps of developing the latent image with toner having the same polarity charge as the latent image; transferring the developed toner from the supporting member to a transfer sheet by pressure-contacting the surface of the supporting member and the transfer sheet with a pressure within the range of from 0.1 to 0.3 kg./cIn. utilizing an electrically conductive roller, the transfer sheet having an electrostatic capacity not greater than the electrostatic capacity of the supporting member and having a volume resistivity within the range of from 10 to 10 lt-cm., the electrically conductive roller and the rear surface of the supporting member having substantially equal potentials; and repeating the developing and transferring steps to produce a plurality of copies from the single electrostatic latent image.

The type of supporting member having the surface which carries the electrostatic latent image is not critical in this invention, so long as it has high electric resistance and is capable of carrying electrostatic latent images. For example, optional photosensitive materials for electrography can be used as the supporting member. The most conventionally used photosensitive members for electrography are formed of conducting base coated with a photoconductive layer. This type of material is useful also in this invention.

As the photoconductive materials, such inorganic substances as selenium, zinc oxide, cadmium sulfide, cadmium-zinc sulfide, cadmium tellurilde (CdTe), selenium telluride (SeTe), cadmium selenide (CdSe), and antimony trisulfide (Sb S and organic photoconductive substances such as anthracene, anthraquinone, and polyvinyl carbazole, can be used. Those photoconductive substances can themselves serve as the photoconductive layers. For example, selenium or cadmium sulfide may be deposited by vacuum evaporation on a suitable substrate or base, or a photoconductive resin such as polyvinyl carbazole can be coated on a substrate to provide the photoconductive coating.

Or, the photoconductive substance may be dispersed in a suitable binder to be applied onto a conducting substrate. As the binder, resinous binders may be used as well as inorganic binders such as Water glass. Useful reinous binders include sytrene polymers and copolymers, polyvinyl acetate and copolymers thereof, acrylic resin, polyvinylacetal and copolymers thereof, polyvinyl alcohol,

3 polyolefins and copolymers thereof, alkyd resin, polyester resin, silicon resin, opoxy resin, and synthetic rubber, etc. Suitable binders are disclosed in British Pat. No. 1,020,506.

Those photoconductive substances may be treated with optional sensitizing dyestuff such as Rose Bengel, Methylene Blue, etc., to be imparted with increased photosensitivity to the light source, or, they may be activated with such metals as gold, copper, etc. Furthermore, they may be surface-treated With such substances as Lewis acid, fatty acid, or metallic salts of the foregoing, organic phosphates, etc., so that the properties of photoconductive layers such as pre-exposure effect (fatigue effect), dark resistance, etc., should be improved.

As the conductive base plates to support the photoconductive layer, besides metallic plates such as of aluminum, copper, and zinc resin plates deposited by vacuum evaporation, or plated with, metal; paper sheets coated with conductive resin, hygroscopic salt, or other conductive materials; and base plate daubed with mixtures of metallic powder with resin, etc.; may be used.

The photoconductive layer is used in the repetitive reproduction according to the process of the invention, in the form of an integrated laminate on the conductive surface of a fiat or cylindrical base.

Still other forms of photosensitive substances employed for electrophotography may also be used in this invention; for example, an overcoated photosensitive member composed of a conductive base, a photoconductive intermediate layer and an insulating surface layer; or of a conductive base, a photoconductive layer, an insulating surface layer, and an intermediate insulating layer provided in the photoconductive layer or between the conductive base and photoconductive layer; may be used. The insulating surface layer can be formed of a film of transparent dielectric substance of high dielectric strength, for example, polyester, cellulose ester, polystyrene, polyolefin, etc.

Electrostatic latent-images can be formed on the surface of those photosensitive materials for electrography by means known per se.

According to the invention, it is also possible to replace those photosensitive materials for electrography with dielectrics of high dielectric strength, the electrostatic latent images being formed on their surfaces by such process as transfer of electrostatic image. The electrostatic latent images on the surfaces of such insulating materials can be likewise used for the repetitive transferring of toner images.

Hereinafter the invention will be explained in fuller details, with reference to the attached drawings.

FIGS. 1 through 3 are explanatory views of each step constituting the process of the invention. FIG. 1 shows a photosensitive member for electrography carrying an electrostatic latent image. FIG. 2 illustrates the procedure of developing the photosensitive member of FIG. 1 with toner, and FIG. 3, that of transferring the toner image on the photosensitive member to the transfer member. FIG. 4 is a diagram showing the arrangement of an apparatus useful for practicing the subject process.

According to the invention, first a surface carrying an electrostatic latent image is developed with toner composed of electrically charged particles of the same polarity as the latent image. Referring to FIG. 1, a photosensitive member 1 for electrography is composed of a conductive base 2, a photoconductive intermediate layer 3, and an insulating surface layer 4, and possesses an electrostatic latent image 5 of a given polarity (negative). When the surface of the member 1 is developed with toner composed of electrically charged particles Got" the same polarity as the latent image 5, as shown in FIG. 2, the toner particles 6 are repelled by the latent image 5 of same polarity and adhere to the portions where the electrostatic latent image 5 is absent. Thus a toner image which is reversed from the electrostatic latent image 5 is formed on the surface carry ing the latent image 5. As the developer, any powder, liquid, or mist developers can sensed. Also optional developing systems, such as magnetic brush methods, cascade processes, powder cloud processes, wet developing methods, mist developing methods, etc. may be employed.

According to the invention, a specific transfer member 7 is pressed against the surface of photosensitive memher 1 carrying the electrostatic latent image 5 and the toner image 6, which is a reverse of latent image 5, by the action of a conducting roller 10 having substantially equal potential to that of the conductive base 2 at the back of the photosensitive surface, as'illustrated in FIG. 3. It is particularly important for the transfer member employed in this invention to possess an electrostatic capacity not greater than that of the supporting member carrying the electrostatic latent image and toner image, andto have a volume resistivity within the range of 1.0 to 10 Q-cm, If the electrostatic capacity of the transfer member is greater than that of the supporting member carrying the electrostatic latent image and toner image, the tendency for the latent image on the surface of the supporting member to disappear upon transfer is enhanced. Also, when the volume resistivity of the transfer member is greater than 10 Q-cm., it is diflicult to transfer the toner image onto the member by the pressing operation alone. Whereas, if the resistivity is less than 10 Q-cm., again the electrostatic latent image on the surface of the supporting member tends to disappear.

According to the invention, by the selective use of the transfer member possessing the specified range of volume resistivity and an electrostatic capacity not greater than that of the supporting member carrying the electrostatic latent image and toner image, the toner image only can be effectively and selectively transferred onto the transfer member, with substantially no dissipating or disturbing of the electrostatic latent image on the surface of the supporting member.

A transfer member Which can be advantageously used for the purpose of this invention is high quality paper having the above-specified characteristics. We further discovered that paper coated with relatively hydrophilic, high molecular weight polymer such as polyvinyl acetate and inorganic filler such as silica, is particularly suitable for the purpose of this invention. Conventional resin-coated paper, insulating paper, etc. exhibit greater tendency to take the electrostatic latent image away from the surface of supporting member and, therefore, are inappropriate for the purpose of this invention. The transfer members particularly advantageous for use with this invention are those having a volume resistivity ranging from 10 -10 Q-cm., and athickness ranging from 50 to 200 Also the electrostatic capacity of the transfer member is, generally speaking, preferably not more than one-tenth that of the latent image-supporting member.

According to the invention the above-specified transfer member is pressure-contacted with the surface of the supporting member carrying the electrostatic latent image and toner image by the conductive roller having substan tially equal potential to that of the back of the supporting member. The conductive roller may be madeof metal or conductive elastomer. The volume resistivity of the conductive roller is preferably no more than 10 .Q-cm. For more intimate contact of the transfer member with the toner image-carrying surface, normally the use of a conductive rubber roller of the degree of hardness ranging from 20-60 is preferred.

Again referring to FIG. 3, the conductive roller 10 consists of a metal axle 11 and a conductive rubber roller 12 mounted on the periphery of the axle, and roller 10 is grounded together with the conductive base 2 at the back of the photosensitive member 1 through the connections 9 and 8. The transfer member 7 is pressure-contacted with the surface of the photosensitive member by the conductive roller 10, and then the two are separated by peeling whereupon the greatest part of the toner particles 6' on the insulating surface layer 4 of the photosensitive member 1 is transferred onto the sheet 7, leaving a minor part of the toner particles 6" on the insulating surface layer 4. In that occasion, the electrostatic latent image 5 on the insulating surface layer 4 remains substantially intact.

The reason for this selective transfer of the toner image is not entirely understood, but our theory is as follows. When the transfer member is pressure contacted with the surface of the photosensitive member by the conductive roller of equal potential to the back of photosensitive member, the intensity of the electric field at the toner adhering portion (no electrostatic latent image-carrying portion) becomes higher at the side of transfer member 7 than that at the side of insulating surface layer 4 because the electric resistance of member 7 is far less than that of the insulating surface layer 4. Consequently the toner image 6' is transferred to the side of member 7. On the other hand, because the electrostatic capacity of transfer member 7 is less than that of the photosensitive member 1, the electrostatic latent image 5 substantially remains intact on the insulating surface layer 4 of photosensitive member 1.

The toner image which has been transferred onto the member 7 can be fixed by any well known means, resulting in a copy sheet. After the toner image is transferred, the electrostatic latent image-carrying surface is again developed with toner particles and the developed toner image is transferred, and thus the so far described developing and transfer processes are repeated. Thus according to the invention, as many as twenty or more copy sheets can be produced from a single electrostatic latent image, with excellent clarity. Furthermore, the process requires neither the application of transfer potential nor the accompanying complex operations such as controlling the potential.

The subject process is readily applicable to repetitive copying or printing, using the equipment of the example illustrated in FIG. 4.

Referring to FIG. 4, photoconductive and photosensitive layer 3 and insulating surface layer 4 are laminated on the cylindrical conductive base plate 2. The photosensitive drum rotates in the direction of the arrow in the drawing to successively pass the various treating stations provided around the drum. The conductive base plate 2 is grounded by the connection 8.

First, the drum surface 4 is subjected to positive or negative DC corona discharge effected by the corona discharging device 13 in the dark, and then to the action of adjacently positioned exposure device 15, whereupon the image isexposed. Simultaneously with the exposure, the surface 4 receives an AC corona discharge or a DC corona discharge of opposite polarity from that of the primary electrical charge, from the corona discharge device 14. Then the entire surface 4 is uniformly exposed to the light from source 16, and an electrostatic latent image is formed on the drum surface. This latent image is developed by the toner 6 having a charge of the same polarity as the electrostatic latent image, by the magnetic brush 18 in the developing device 17.

The transfer member 7 is held between the conductive roller and the photosensitive drum and unwound from a supply reel 19. The conductive roller 10 is grounded by connection 9, and is given the same electric potential as with that of the conductive base plate 2. In short, the transfer member 7 is dried before its use in the transfer process, by a pair of heating rollers 20, and then is pressure-contacted with the drum surface by the conductive roller 10 whereat the selective transfer of toner image is performed. The transfer member 7 then carrying the toner image is led to the fixing device 21, and the member having the fixed toner image is wound up by a take-up reel 22.

A cleaning device 23 is mounted on a rotating shaft 24, and is located at the position 23 spaced from the photosensitive drum, when the electrostatic latent image-forming station I composed of the corona discharge devices 13 and 14, optical system 15, and light source 16, is not operating.

During the repetitive transfer, all the actions of the electrostatic latent image-forming station I and the cleaning station C are suspended, and only the developing station D and transfer station T are operated. Thus the photosensitive drum from which the toner image has been removed by the transfer process and which therefore retains the electrostatic latent image alone is rotated, again deposited with the toner particles in the developing system D, and the image is transferred in the transfer station T. By this repetition of the above-described development and transfer, any desired number of copies can be obtained.

For the formation of a new electrostatic latent image, the cleaning station C and image-forming station I are operated. 7

According to the process of the invention, the generally preferred rate of toner image (traveling speed of the transfer member) is at least 20 cm./sec., and the pressure exerted at the time of transfer is approximately 0.1-0.3 kg./cm. Lower humidities are preferred for the transfer operation, such as not higher than 60% relative humidity. Obviously, the repetitive transfer is not inoperable at humidities higher than the above-specified level, but in that case it is desirable to pre-heat the transfer member to be employed with a roll heater, etc., to remove its moisture content in advance.

Hereinafter the invention will be explained more specifically, with reference to the following examples.

EXAMPLE 1 The surface of an overcoated photosensitive member composed of a transparent dielectric surface layer, cadmium sulfide photoconductive layer, and a conductive base, having an electrostatic capacity of 500 ,uf./cm. and a volume resistivity of 3.3 X10 Q-cm. was subjected to a corona discharge of +7 kv. to be uniformly charged in positive polarity. Then an optical image was radiated for approximately 0.5 second, with an illumination intensity of 10 lux, simultaneously with another corona discharge of -7 kv., to form an electrostatic latent image on the surface. The surface was developed with a negatively charged toner, i.e., the same polarity as that of the surface electric charge forming the electrostatic latent image. Separately, a transfer member composed of a sheet of approximately thick high quality paper having an electrostatic capacity of 350 ,uf./cm. and a volume resistivity of 10 from. with one of its surfaces coated with a 10% aqueous solution of 5:1:2 mixture (weight ratio) of silica:vinyl acetate:acrylonitrile-styrene copolymer was prepared. The powder image was intimately contacted with the coated surface of the transfer sheet, and the contacting portion was subjected to a pressure of 0.2 kg./cm. exerted from the back of the transfer sheet, by a conductive rubber roller of 45 in degree of hardness. The transfer member was traveled at a rate of 30 cm./ sec., and as soon as the conductive rubber roller left the transfer sheet, the sheet was peeled off from the overcoated photosensitive member to produce the first copy.

The overcoated photosensitive member after the first powder image transfer as above-described was again developed with the developer in the similar manner, without intervening the cleaning of the residual powder and electric charge as conventionally practiced in the art of Ielectrography, nor the electrostatic latent image-forming steps such as charging and optical image irradiation, etc., whereupon an image of substantially equal clarity with that of the first-formed powder image was obtained. The powder image was similarly transferred onto the transfer sheet to produce the second copy. Thereafter the development and transfer procedures were simply repeated without any additional treatment, to produce on the average ten copies in an atmosphere of 60% in relatively humidity.

Also when the transfer sheet was heated and removed of the moisture content in advance of its contact with the photosensitive plate carrying the powder image, on the average twenty sheets of the duplicate Were'obtained.

Such repetitive transfer was experimented in a similiar manner using various types of transfer sheets with the results as in the table below.

The repetitive transfer was practiced in a manner similar to Example 1, using a high quality paper transfer sheet coated with a silica-vinyl acetate, mixture, in an atmosphere of 90% in relative humidity. Thus 2 copies were obtained.

The above experiment was repeated, except that the transfer sheet was passed between roll heaters heated to approximately 105 C. immediately before its contact with the photosensitive plate carrying the toner image. Whereupon ten good copies were obtained.

EXAMPLE 3 Under the conditions identical With those of Example 2, the transfer process was experimented, using untreated, 75 thick high quality paper as the transfer sheet. Only one copy was producible when the pre-drying with roll heaters was omitted, but when the sheet was pretreated with the 150 C. roll heaters, five good copies were obtained.

What we claim is:

1. A transfer process for producing a plurality of copies from a single electrostatic latent image formed on the surface of a supporting member comprising the steps of developing the latent image with toner having the same polarity charge as the latent image; transferring the developed toner image from the supporting member to a transfer sheet without substantially dissipating the electrostatic latent image by pressure-contacting the surface of the supporting member and the transfer sheet with a pressure within the range of from 0.1 to 0.3 l g./cm.

utilizing an electrically conductive roller, the transfer sheet having an electrostatic capacity not greater than the electrostatic capacity of the supporting member and having a volume resistivity within the range of from 10 to 10 Q-cm., the electrically conductive roller and the rear surface of the supporting member having substantially equal potentials, and repeating said developing and transferring steps to produce a plurality of copies from the single electrostatic latent image.

2. The process according to Claim 1, in which the volume resistivity of the transfer sheet is within the range of from 10 to 10 from.

3. The process according to Claim 1, in which the thickness of the transfer sheet is within the range of from to ZOO t.

4. The process according to Claim 1, in which the electrostatic capacity of the transfer sheet is not greater than the electrostatic capacity of the supporting member.

5. The process according to Claim 1, in which the volume resistivity of the conductive roller is not greater than 10 il-crn.

6. The process according to Claim 1, in which the degree of hardness of the conductive roller is within the range of from 20 to 60.

7. The process according to Claim 1, in which the traveling speed of the transfer sheet is not less than 20 cm./sec.

8. The process according to Claim 1, in which transferring step is performed in an atmosphere of not higher than in relative humidity. I

9. The process according to Claim 1, in which the transfer sheet is heated to remove moisture therefrom prior to contacting the supporting member.

References Cited UNITED STATES PATENTS 3,627,523 12/1971 Shelffo 11717.5 3,594,161 7/1971 Kaufman 117-17.5 2,812,709 11/1957 Gundlach 11717.5 3,045,587 7/1962 Schwertz 101-Dig. 13 3,267,840 8/1966 Honma et a1. 101--Dig. 13 3,363,555 1/1968 Olden 107-Dig. 13 3,084,043 4/1963 Gundlach 117--37 LE 3,712,728 1/1973 Whittaker 11737 LE MICHAEL SOFOCLEO US, Primary Examiner U.S. Cl. X.R.

50 961 SD, 1 LY, 1.4; 101--Dig. 13; 11737 LE UNITED STATES PATENT OFFICE 3 CERTIFICATE OF CORRECTION Patent No. 3,843,381 I Dated 0ctoberj22, 1974 Invent0r( s) SHOJI MATsUMoToET AL It is Certified that error appears in the above-identified patent and: that Said Letters Patent are hereby corrected as shown below:

In the Heading, Correct patentees' Claim to Priority to read as follows: C1aims priority apolication Japan, Dec. 29, 1970,

ClaiTn 2, lint-3'3 thereof: cancel 10 to 10' 'cm." and sub stitute therefor 10 to 10 a cm.

Signed and sealed this 10th day of December 1974.

(SEAL) Attest: v

MCCOYM. GIBSON JR. c. MARSHALL DANN I Attesting Officer Commissioner of Patents FORM PO-1050 (10-69) USCOMNFDC 376m w u. s, covsmmzm rum-nus omc: nu o-au-au, 

1. A TRANSFER PROCESS FOR PRODUCING A PLURALITY OF COPIES FROM A SINGLE ELECTROSTATIC LATENT IMAGE FORMED ON THE SURFACE OF A SUPPORTING MEMBER COMPRISING THE STEPS OF DEVELOPING THE LATENT IMAGE WITH TONER HAVING THE SAME POLARITY CHARGE AS THE LATENT IMAGE; TRANSFERRING THE DEVELOPED TONER IMAGE FROM THE SUPPORTING MEMBER TO A TRANSFER SHEET WITHOUT SUBSTANTIALLY DISSIPATING THE ELEC TROSTATIC LATENT IMAGE BY PRESSURE-CONTACTING THE SURFACE OF THE SUPPORTING MEMBER AND THE TRANSFER SHEET WITH A PRESSURE WITHING THE RANGE OF FROM 0.1 TO 0.3 KG./CM.2 UTILIZING AN ELECTRICALLY CONDUCTIVE ROLLER, THE TRANSFER SHEET HAVING AN ELECTROSTATIC CAPACITY NOT GREATER THAN THE ELECTROSTATIC CAPACITY OF THE SUPPORTING MEMBER AND HAVING A VOLUME RESISTIVITY WITHIN THE RANGE OF FROM 10**8 TO 10**14 $.CM., THE ELECTRICALLY CONDUCTIVE ROLLER AND THE REAR SURFACE OF THE SUPPORTING MEMBER HAVING SUBSTANTIALLY EQUAL POTENTIALS, AND REPEATING SAID DEVELOPING AND TRANSFERRING STEPS TO PRODUCE A PLURALITY OF COPIES FROM THE SINGLE ELECTROSTATIC LATENT IMAGE. 